Abstract
Pyrogenic carbon (PyC) is an important component of the global soil carbon (C) pool, but its fate, persistence, and loss dynamics in contrasting soils and environments under planted field conditions are poorly understood. To fill this knowledge gap, a 13C-labelled biochar, as a surrogate material for PyC, produced from Eucalyptus saligna by slow pyrolysis (450°C; δ13C -36.7‰) was surface (0−10 cm) applied in C3 dominated temperate pasture systems across Arenosol, Cambisol and Ferralsol. The results show a low proportion of the applied biochar-C mineralised over 12 months in a relatively clay- and C-poor Arenosol (i.e., 2.0% loss via mineralisation), followed by a clay- and C-rich Cambisol (4.6%), and clay-, C- and earthworm-rich Ferralsol (7.0%). The biochar-C mean residence time (MRT), estimated by different models, varied between 44−1079 (Arenosol), 18−172 (Cambisol), and 11−29 (Ferralsol) years, with the shorter MRT estimated by a one-pool exponential and the longer MRT by an infinite-pool power or a two-pool exponential model. The two-pool model was best fitted to biochar-C mineralisation. The biochar-C recovery in the 12−30 cm soil layer varied from between 1.2% (Arenosol), 2.5−2.7% (Cambisol) and 13.8−15.7% (Ferralsol) of the applied biochar-C after 8−12 months. There was a further migration of biochar-C below the 50-cm depth in the Arenosol, as the combined biochar-C recovery in the mineralised pool and soil profile (up to 30 or 50 cm) was 82%, in contrast to 101% in the Cambisol and 104% in the Ferralsol after 12 months. These results indicate that the downward migration of biochar-C was greatest in the Arenosol (cf. Cambisol and Ferralsol). Cumulative CO2-C emission from native soil-plant sources was lower (p <0.10) in the biochar-amended vs. non-amended Ferralsol. This field-based study shows that the downward migration of biochar-C exceeded its loss via mineralisation in the Arenosol and Ferralsol, but not in the Cambisol. It is thus important to understand biochar-soil interactions to maximise long-term biochar C sequestration potential in planted soil systems.
Highlights
Pyrolysed carbon (PyC) is produced naturally during fires following incomplete combustion of biomass [1] and is an important but poorly understood pool of the global carbon (C) cycle
The total C emission rates were similar in the biochar-amended and control micro-plots throughout, except in the Arenosol where greater C emission rates were observed in the biochar amended vs. control micro-plots during the first 1.5-months (Fig 1)
Compared to the relatively long mean residence time (MRT) estimated by Fang et al [22], this field-based study estimated far shorter MRT of biochar-C using the twopool exponential model. These results demonstrate that field MRT estimates cannot be extrapolated from laboratory studies, as proposed by Kuzyakov et al [24, 62] who suggested that a 10 times increase in MRT of biochar-C might be expected in field conditions of lower temperature and microbial activity compared to controlled laboratory studies
Summary
Pyrolysed carbon (PyC) is produced naturally during fires following incomplete combustion of biomass [1] and is an important but poorly understood pool of the global carbon (C) cycle. Understanding the fate, persistence and reaction of PyC in soil is crucial for constraining C models to accurately predict PyC sequestration potential and evaluate its contribution to the global C budget. Biochar is a form of PyC produced intentionally via pyrolysis under controlled conditions from plant biomass or bio-waste. Biochar may be a good surrogate for natural PyC as it has similar, or greater, C sequestration characteristics [9, 10]. Biochar could improve our understanding of the fate and persistence of PyC and its influence of native plant-soil organic C in terrestrial ecosystems
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